Conventionally, it is required to form an ohmic electrode as a drain electrode, for example, in a vertical power device having a silicon carbide substrate (i.e., SIC substrate). The ohmic electrode connects the power device and an electric circuit such that a contact resistance between the SIC substrate and the ohmic electrode such as the drain electrode is reduced.
A method for forming the ohmic electrode is such that an impurity doped layer is formed by an ion implantation process, and the impurity is activated. In this case, a backside electrode is formed by a following process. First, an electrode is formed on a front surface of the SIC substrate having a vertical semiconductor device. Then, a resin film is formed to cover the surface of the SiC substrate, and a thin film is formed on the backside of the SiC substrate. An impurity is implanted on the backside of the SIC substrate. Then, a laser beam is irradiated on the backside of the SIC substrate. After that, a metal film is formed on the backside of the SIC substrate, so that the electrode is formed on the backside.
However, it is necessary to anneal the substrate for a long time in the ion implantation process. Further, the ion implantation apparatus is expensive, and further, the cost of an ion implantation process is very high. Accordingly, it is required to form the ohmic electrode without performing the ion implantation process.
It is considered as a method for forming the ohmic electrode without performing the ion implantation process that a metal film is formed on the SiC substrate, and a laser beam is irradiated on the SIC substrate. In this case, the backside electrode is formed by a following process. First, the backside of the SIC substrate is ground so that a convexity and concavity is formed on the backside with a surface roughness (i.e., Ra) equal to or larger than 10 nanometers and equal to or smaller than 500 nanometers. Then, the metal film is formed on the backside of the substrate. After that, the laser beam is irradiated on the backside of the SIC substrate.
However, in the above method, it is necessary to perform a grinding process for roughening the backside of the SiC substrate, so that the backside of the substrate is roughened. Thus, the backside may be damaged, and therefore, a crack may be generated in the substrate. An electric property and a mechanical property of the substrate may be deteriorated. Accordingly, it is requested to form the ohmic electrode without grinding the backside to form the convexity and concavity.
In view of the above points, JP-A-2007-96263 (corresponding to US 2007/0045631-A1) teaches a method for terminate the backside of the SIC substrate with a hydrogen atom or a hydroxyl group so as to easily displace an electron. The backside of the substrate provides a contact boundary with the ohmic electrode.
However, when the backside of the substrate is terminated with the hydrogen atom or the hydroxyl group as described in JP-A-2007-96263. and a sintering temperature of the ohmic electrode is equal to or higher than 800° C., the hydrogen atom and the hydroxyl group are removed from the substrate. Thus, the termination effect is disappeared, so that the ohmic property is not obtained.